1. Field of the Invention
This invention relates generally to patterned perpendicular magnetic recording media, such as disks for use in magnetic recording hard disk drives, and more particularly to patterned disks with data islands having improved magnetic recording properties.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In conventional continuous magnetic recording media, the magnetic recording layer is a continuous layer over the entire surface of the disk. In patterned media, also called bit-patterned-media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. Perpendicular media will likely be the choice for patterned media because of the increased data density potential of perpendicular media. To produce magnetic isolation of the patterned data islands, the magnetization of the spaces between the islands is destroyed or substantially reduced to render these spaces essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the spaces between the islands.
In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define troughs or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with relatively low-cost, high volume processes such as lithography and nanoimprinting. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. The trenches may be recessed far enough from the read/write head so as to not adversely affect reading or writing, or the magnetic material in the trenches may be rendered essentially nonmagnetic during fabrication. This type of patterned media is described by Moritz et al., “Patterned Media Made From Pre-Etched Wafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp. 1731-1736.
One problem associated with patterned perpendicular media is the relatively wide variation in the coercive field (Hc) among the individual magnetic islands, sometimes also called “dots”. This variation is characterized by a wide distribution of the switching field, i.e., the write field required to switch the magnetization of a magnetic island from one state to the other state. Ideally the switching field distribution (SFD) width would be zero, meaning that all the bits would switch at the same write field strength. A high-width SFD decreases the bit-addressability because the likelihood of the write field switching the magnetization of dots adjacent to the dot being addressed is increased. Additionally, it has been found that the SFD broadens (that is, the bit-to-bit variation in the coercive field increases) as the size of the magnetic islands is reduced, which limits the achievable density of patterned perpendicular media. The SFD in arrays of dots with perpendicular magnetic anisotropy has been explained by a distribution of intrinsic anisotropy by Thomson et al., “Intrinsic Distribution of Magnetic Anisotropy in Thin Films Probed by Patterned Nanostructures”, Phys. Rev. Lett. 96, 257204 (2006).
An additional problem arises in patterned perpendicular media because of the ultra-high density of the dots. In conventional continuous perpendicular media, the linear density (along-the-track density of the dots) is typically 4 to 6 times the track density (the density of the tracks in the radial or cross-track direction), while in patterned media the linear density and track density will be about the same. This is because any suitable patterned media fabrication process will only be utilized to its full potential if along-the-track and cross-track dot spacings are similar in size. Thus, much higher track densities are anticipated for patterned perpendicular media. However, due to the high write field and limited head field gradients achievable there may be fringing fields from the write head leaking into adjacent dots in adjacent tracks. This can cause inadvertent switching of the magnetization (overwriting) of adjacent dots. Also, as the linear density increases, fringing fields from the write head may also cause inadvertent switching of adjacent dots in the same track as the dot being addressed. This problem is exacerbated because the fringing fields acting on adjacent dots encompass relatively large angles with the perpendicular easy-axis of the recording layer on the dots, which increases the likelihood of overwriting.
There is also a need in patterned perpendicular media for a higher readback signal from the individual dots and thus a higher signal-to-noise ratio (SNR) in the readback signal.
What is needed is a patterned perpendicular magnetic recording medium that has increased readback signal from the dots and improved bit-addressability during writing so as to be less susceptible to overwriting of adjacent dots.